This invention relates to a power steering device, and more particularly to a power steering device suitably used for a forklift and adapted to hold a steering angle when an operator looses his hold from a handle.
Conventionally, a vehicle such as a forklift or the like is generally mounted thereon with a power steering device of the full-hydraulic type generally constructed in such a manner as shown in FIG. 8. More particularly, the conventional power steering device includes a piston 101, which is operatively associated or connected at both ends with or to wheels (not shown) and arranged in a cylinder 102 in a manner to be slidable therein. The cylinder 102 has cylinder chambers 102a and 102b defined therein with the piston 101 being interposed therebetween. The cylinder chambers 102a and 102b are connected to cylinder ports 104a and 104b of a change-over valve 103.
The change-over valve 103 is provided with a pair of communication ports 105a and 105b, which are commonly connected to a trochoidal pump 106. The trochoidal pump 106 is operatively associated with a handle (not shown). When the handle is turned, the trochoidal pump functions to introduce fluid through a pressure port 106a or 106b thereinto and discharge fluid in an amount proportional to the number of turnings of the handle through the pressure port 106b or 106a.
Also, the change-over valve 103 includes a pump port 107 connected to a pump P and a tank port 108 connected through a filter 109 to a tank T.
In the change-over valve 103 thus constructed, the cylinder ports 104a and 104b are kept closed when the valve 103 is at a neutral position shown in FIG. 8.
The change-over valve 103 may comprise a rotary valve associated with the handle. The valve 103 may be constructed so as to be changed over depending on torque inputted thereto from the handle and kept at the neutral position unless torque is inputted thereto from the handle.
When the handle is operated to change over the change-over valve 103 to a change-over position 103a, the pump port 107 is permitted to communicate with the communication port 105a, so that fluid may be introduced from the pump P to the trochoidal pump 106. Then, the trochoidal pump 106 discharges fluid in an amount proportional to the number of turnings of the handle from the pressure port 106b.
Fluid discharged from the trochoidal pump 106 through the pressure port 106b is guided to the communication port 105b. The change-over position 103a permits the communication port 105b to communicate with the cylinder port 104b, so that fluid discharged from the trochoidal pump 106 is fed to the cylinder chamber 102b of the cylinder 102.
Also, the the change-over position 103a permits the cylinder port 104a connected to the chamber 102a of the cylinder 102 to communicate with the tank port 108, so that fluid in the chamber 102a is discharged to the tank T.
Such feeding of fluid to the chamber 102b and discharge of fluid in the chamber 102a lead to movement of the piston 101, to thereby generate power assisting force for steering of the wheels.
When an operator looses his hold of the handle while keeping the wheels at a position thus steered, the change-over valve 103 is held at a neutral position. Such holing of the valve at the neutral position causes the cylinder ports 104a and 104b to be closed, so that fluid is confined in the chambers 102a and 102b. This results in the steered position of the wheels being maintained against self aligning torque, to thereby permit the wheels to maintain a steering angle thus obtained even when any external force is applied to the wheels.
When the handle is turned in a reverse direction, the change-over valve 103 is changed over to a change-over position 103b and fluid is discharged through the pressure port 106a of the trochoidal pump 106. The fluid thus discharged is then fed to the chamber 102a of the cylinder 102 and fluid in the chamber 102b is discharged to the tank T.
In the conventional power steering device thus constructed, the trochoidal pump 106 associated with the handle is arranged for the purpose of metering the amount of fluid fed to the cylinder 102.
Unfortunately, such a trochoidal pump inherently causes leakage of fluid therefrom due to its inherent structural defect. When it is not possible to carry out steering of the wheels as seen during rest swing locking, the fluid leakage affects the handle, to thereby cause turning of the handle to continue.
Thus, the conventional power steering device of the full-hydraulic type often causes the handle to be turned independently from the wheels. This leads to misregistration between a neutral point of the handle and that of the wheels, so that an operator often fails to appropriately judge the neutral position of the handle based on a position of the handle.
Also, in the conventional power steering device, turning of the handle causes fluid in an amount proportional to the amount of turning of the handle to be suddenly fed from the trochoidal pump 106 to the cylinder 101, resulting in a feeling of the steering at the time when application of assisting force is started being deteriorated.